Stannous chloride (SnCl_2) has the disadvantages of being easily hydrolyzed and oxidized, but it is included in most commercial kits as an appropriate reducing agent in the preparation of ^<99m>Tc radiopharmaceuticals. The replacement of SnCl_2 by the insoluble macromolecular Sn (II) (R-Sn) complex was proposed as a way to resolve some of the problems caused by a large excess of SnCl_2 Macroreticular chelating ion exchange resins having an adsorption ability for Sn (II) were investigated for the development of R-Sn complex. Among them, a chelating resin containing aminophosphonic acid groups showed a high capacity for Sn (II) , which bound strongly to the resin by chelation. The use of the R-Sn complex was expected to minimize Sn (II) comtamination of the ^<99m>Tc labeling solution and be effectively used as a reducing agent for ^<99m>Tc labeling of proteins containing sulfhydryl groups. So, the R-Sn complex was applied to the direct ^<99m>Tc labeling of human immunoglobulin (IgG) to minimize the influence of Sn (II). ^<99m>Tc labeling was achieved at greater than 90% yield simply by the short-term mixing of IgGa containing>2 -SH groups per IgG molecule, ^<99m>Tc pertechnetate, and the R-Sn complex in pH 7 solution. The ^<99m>Tc-IgGa obtained by this labeling method has high stability based on the thiol-specific binding of ^<99m>Tc without transchelation from another weakly bound ^<99m>Tc-complex.このR-Snを抗体中のジスルフィドの還元処理によって生成するSH基を利用する抗体の^<99m>Tc直接標識に応用した。なお、R-Snは、抗体中のジスフィルドを還元しないため、SH基生成のための還元剤として、ジチオスレイトールを使用した。2個以上のSH基を有する抗体であれば、pH7のリン酸緩衝液中で、R-Sn及び^<99m>TcO^-_4と短時間の振とうによる簡便な操作だけで、90%以上の収率で標識が可能であった。さらに、この標識法により調整された^<99m>Tc標識抗体は、^<99m>TcがSH基に選択的に結合することによってもたらされる高い安定性を有しており、多様なモノクローン抗体の標識に応用することが可能である。